This project will test the hypothesis that specific neurons in three neuroanatomical sites important in the ventilatory CO2 chemoreflex also participate in the regulation of sympathetic nerve activity (SNA) and blood pressure (BP) in normal rats and mice, and in Spontaneously Hypertensive Rats (SHR). The three central chemoreceptor sites (and neurons) are: 1) orexin neurons of the lateral hypothalamus (LHA); 2) Phox2b- expressing neurons of the retrotrapezoid nucleus (RTN); and 3) serotonergic (5-HT) neurons of the medullary raphe (MR). The measurements, obtained in unanesthetized rodents during wakefulness, REM and NREM sleep include ventilation, SNA and BP at rest while breathing air and in response to breathing 5% CO2. The experiments involve focal disruption of: 1) orexin neuron function by siRNA induced inhibition of prepro-orexin in the lateral hypothalamus, 2) Phox2b-expressing neurons of the RTN by viral transfection of a PRS8/allatostatin receptor construct followed by subsequent allatostatin injection to inhibit the neurons reversibly; 3) 5-HT neuron function in: a) mice by transgenic insertion of the inhibitory HM4D receptor in all 5-HT neurons or in 5-HT neurons derived from rhombomeres 3 and 5 (DREADD: Designer Receptor Activated by Designer Drug), and b) rats by siRNA/shRNA to block activity of tryptophan hydroxylase 2 the enzyme for 5-HT synthesis (TPH2). In addition, the experiments involve antagonism of orexin receptors by almorexant administered systemically and focally within the medullary raphe and retrotrapezoid nucleus. The major goals are to further the understanding of the link between chemoreception and blood pressure regulation, and to begin to relate this understanding to the development of some forms of hypertension, which has relevance to the mission of the agency and to medical practice.